The United States consumes approximately thirty percent of the world's annual energy supplies. About ninety-three percent of our energy requirements are provided by fossil fuels, of which nearly half are from crude oil sources. Since crude oil production in the United States has been falling off since reaching a peak in 1972-1973, crude oil is supplied largely from the Middle East. Recent events in the Middle East have sharply focused attention on our overdependence on foreign crude oil supplies and have made apparent our need to develop alternate energy sources. Thus, our attention has focused on finding renewable or noncritical forms of energy to replace our current foreign crude oil supplies. Perennial growth matter such as wood, cotton trash, corn stalks, wheat chaff, forest residues, alfalfa, sunflower stalks, weeds, leaves, and other similar vegetation are in the category of renewable energy sources. Coal, because of its abundance here in the United States, is considered a noncritical energy form. The widespread utilization of both the renewable and noncritical energy supplies has been hampered by a lack of suitable means by which we can directly convert these energy supplies to useful work in such applications as automobiles, aircraft, locomotives, trucks, buses, electric generators, pumps, etc. A most convenient energy conversion device with multi-fuel capabilities is a gas turbine engine.
The direct injection of solid fuels into gas turbine combustion systems requires proper preparation of the fuels prior to burning and the removal of combustion residues, which have a detrimental affect on the turbine blades. In order to burn solid fuels in gas turbine systems, it has been found that the solid fuels must be ground, shredded or pulverized to an appropriate powder size.
The direct injection of solid fuels into gas turbine combustion systems dictates that the solid fuels be burned quickly and completely. It has also been found that considerable complexity and expense is incurred in the removal of combustion residues in order to protect turbine blades from abrasion. This is particularly true if coal is employed as a fuel in a gas turbine engine. It has been found that it is only necessary to remove the larger residues because solid particles of micron size have minimal influence on turbine blade abrasion.
Several prior art designs have unsuccessfully attempted to solve the problem of solid fuel combustion and the effective removal of large combustion residues from the hot gas flow field so as to mitigate turbine blade erosion.
In U.S. Pat. Nos. 2,625,791 and 2,651,176 are described gas turbine combustion systems utilizing solid fuel injection as well as the removal of the harmful solid residues from the products of combustion. In each design, the combustion system includes a pressurized fuel inlet for comminuted fuel incorporating a pneumatic pulverizing device and preheating means for simultaneously heating the fluidized solid fuel stream downstream of the pneumatic pulverizing device and adding a supplementary accelerated stream of heated air to the solid fuel stream. This air and fuel mixture is then discharged into a combination preheater and cyclonizer to effect further disintegration and devolutilization of the solid fuel. From the cyclonizer, this combustible fluid is injected into the center of a vertically disposed combustor. Combustion air is fed tangentially into the combustor through two feed zones. The first feed zone is designed to provide a quantity of air sufficient to completely burn the fuel injected into the combustor. The second feed zone is designed to reduce the temperature of the products of combustion, including the residues, to an operative optimum for the turbine blades. Downstream of the combustor, a discharge pipe with a foraminous screen is covered with a deflector plate to form a residue separator. Further downstream of this residue separator is a battery of small cyclone separators for discharging clean combusted air into the turbine section of the gas turbine.
In U.S. Pat. Nos. 2,888,804 and 2,911,065 are described further improvements in a gas turbine combustion system utilizing solid fuel injection and solid residue removing equipment. In each design, the combustion system therein described is a horizontally disposed combustor with a horizontally disposed fuel injector, which is concentrically mounted in the combustor. The fuel injector includes an inner fuel oil supply line and a nozzle, an annular air duct surrounding the fuel oil supply line and nozzle and a third annular duct wherein airborne pulverized coal is discharged tangentially into the annulus between the second and third annular duct. In the combustor, combustion air is staged into the combustor to mix with the fuel emitted from the fuel injector to form a fluidized gaseous stream containing combustion residues. Downstream of the fuel injector and mounted to the end of the combustor, is a terminal mixing and discharge section whereby the fluidized gaseous stream is diluted with cooler air to lower the temperature of the fluidized gaseous stream to the optimum turbine operating temperature. The fluidized gaseous stream and the combustion residues are then discharged from the combustor and are projected against a curveform louvered upper surface of a plenum chamber. Cooler air flows through the louvered opening to quench the combustion residues impinging against it. The fluidized gaseous stream and combustion residues are then passed through a battery of vortex separators, with the result that the combustion residues are trapped, removed, cooled and discharged from the fluidized gaseous stream. The cleaned fluidized gaseous stream is then discharged against the turbine blades.
All of the aforesaid designs were complicated and none were able to quench all of the combustion residues, with the result that the combustion residue separators melted and burned out. In addition, none of the aforesaid designs considered the use of other renewable sources of energy, such as those in the category of perennial growth vegetation.
A more recent design is shown in U.S. Pat. No. 4,089,631, wherein gas-fluidized ground coal and coal dust slurred with fuel oil are supplied to a pressurized slagging reverse flow cyclone combustor. The combustor has a relatively long axial length in order to achieve good separative efficiency of the small particulates from the flow field. In addition, a base purge and a conical vortex shield are incorporated in the combustor to inhibit reentrainment of fly ash into the exiting vortex core. Clean combustion air is admitted centrally into the cyclone combustor while gas-fluidized ground coal is introduced into the cyclone combustor near the cyclone wall in a relatively minor proportion of the total combustion air to produce a hot gas temperature in the range of 2600 degrees to 2900 degrees Fahrenheit. The combustion process is such that the coal particulates, because of their swirling motion, move rapidly outwardly into the wall burning zone of the combustion chamber, and the molten ash in the bottom of the combustion chamber tends to entrain fly ash particles from the wall burning zone of the chamber. This design is large, bulky, and would not be easily incorporated in a mobile gas turbine power plant that must be lightweight and compact. Furthermore, a turbine system incorporating this combustion scheme must operate at very high combustion temperatures in order to form slag, which is required to entrain fly ash particles in order to assist in the clean-up of the combustion gas flow field. In addition, this concept requires using fuel oil in order to entrain small coal dust particles in a fuel slurry to enhance particulate control in the combustor.